Error on qualifying a Lead - Cannot read property 'setValue' of null

On qualifying a lead to a case, information is copied to the Contact and the Case. But every time I qualify, I get this error: Cannot read property 'setValue' of null.  It's strange as the case is created and the updates are there, however, I suspect that one of the fields is not actually being set or updated.  How can I find out what field this is?  The error log is as follows:

TypeError: Cannot read property 'setValue' of null
at Object.success (crm.harboursport.co.nz/.../hs_ReferralJSLibrary
at o (crm.harboursport.co.nz/.../hs_jQueryLib
at Object.fireWith [as resolveWith] (crm.harboursport.co.nz/.../hs_jQueryLib
at w (crm.harboursport.co.nz/.../hs_jQueryLib
at d (crm.harboursport.co.nz/.../hs_jQueryLib
at Object.send (crm.harboursport.co.nz/.../hs_jQueryLib
at Function.ajax (crm.harboursport.co.nz/.../hs_jQueryLib
at ShowSelectedContactDetails (crm.harboursport.co.nz/.../hs_ReferralJSLibrary
at eval (eval at RunHandlerInternal (crm.harboursport.co.nz/.../ClientApiWrapper.aspx, <anonymous>:1:1)
at RunHandlerInternal (https://crm.harboursport.co.nz/form/ClientApiWrapper.aspx?ver=-904332278:157:1)

Thank you!